Authorities allege he was assassinated by two women who wiped the substance on his face while he waited to board a flight at Kuala Lumpur airport on Feb. 13.

South Korean intelligence officials believe this was an assassination plot orchestrated by the North Korean government.

Here's the lowdown on VX.

What is it?

"VX is the most toxic chemical weapon ever produced," according to Hamish de Bretton-Gordon, a former NATO commanding officer and leading chemical weapons expert.

“VX acts so quickly that victims would have to be injected with the antidote almost immediately to have a chance at survival”

It's banned under several international conventions and was designated a weapon of mass destruction by a U.N. resolution in April 1991.

Its origins date back to the early 1950s, when a British scientist named Ranajit Ghosh was researching pesticides and developed the "V-series" of nerve agents — the V stood for "venom."

According to the Council on Foreign Relations, a Washington-based think tank, the compound was deemed too deadly for commercial use.

However, the U.K. shared the formula with the American government, which began full-scale production of VX in 1961.

Who has it?

During the Cold War, both Washington and Moscow built up large quantities of chemical weapons, including VX.

But after signing the Chemical Weapons Convention, which banned the production, stockpiling and use of chemical weapons, the U.S. says it has destroyed all of its arsenal and Russia has pledged to do the same by 2020.

Elsewhere, the spread of VX is believed to be relatively contained, mainly because it takes a sophisticated laboratory to produce.

Saddam Hussein was accused successfully weaponizing VX in the 1980s, before using it against Iranian forces and the Kurds.

A decade later, the Japanese cult Aum Shinrikyo killed 12 people on the Tokyo subway using the less-toxic nerve agent sarin. The group also killed one person using VX.